Think of all the people you see wandering round our big 4 supermarkets filling trolleys- maybe a hundred customers at any one time per store. Order pickers would be quicker but you'd probably still need 50 members of staff at any one time with trolleys. And you'd need a team of maybe 10 admin staff to take the orders, bill them, resolve queries over substituted goods. You'd also need a handover team of maybe 10 people to hand over the packs to make sure everyone gets the right bags. I don't think supermarkets have anything like that number of staff at any one time